Previous research at Stanford University involving automatic aircraft landings using a high-precision form of differential GPS known as Carrier-Phase Differential GPS (CPDGPS) led to the development of a CPDGPS-based sensor system for automatic tractor control. With high accuracy and no drift in attitude (roll, pitch, and yawl and/or position, CPDPGS offers a cost-effective sensor option for automatic guidance systems. Stanford researchers first experimentally demonstrated automatic steering of a medium-sized Deere 7800 tractor in the spring of 1996. Subsequent research included tractor control along spirals, arcs, and arbitrary curves, control on steeply sloped terrain, on-line identification of the steering valve's 'dead-zone', and real-time identification/improvement of the tractor model. Typical controller accuracies under full engine load with implement lowered were near 0 cm mean and 4-6 cm S.D. in the tracking error of the control point on the tractor from the desired trajectory las measured by CPDGPS). Unbiased low-noise attitude measurements were vital because the control point is not collocated with the GPS position antenna: an attitude measurement noise of only 1 degrees (1 sigma) is shown to introduce additional position uncertainty of up to 4 cm at the control point above the original GPS measurement (for a tractor of similar size to the one used in this research). In other words, accurate position information alone is not enough for a viable sensor system. These experimental results, enthusiastically received by farmers who have witnessed Stanford's 'GPS tractor', show that CPDGPS could be the sole position and attitude sensor for a viable commercial tractor guidance system. (C) 2000 Elsevier Science B.V. All rights reserved.
